(1) Assembly Preparation

① Prepare drawings, technical documents, and work instructions according to the production order.

② Based on the assembly batch size, prepare all parts and components at once according to the parts list or bill of materials in the assembly drawing.

③ Re-inspect the main mating dimensions of the parts and components; if assembly is performed using a selective assembly method, group the parts accordingly; check the certificates of conformity for outsourced and purchased parts to ensure that all parts and components entering assembly are qualified products within their effective service life.

④ The stroke adjustment mechanism of the adjustable stroke cylinder should operate smoothly and reliably and meet the accuracy requirements.

⑤ The working surfaces and mating surfaces of the main parts and components are not allowed to have defects such as rust, scratches, or dents. All sealing elements (including dust seals, retaining rings, support rings, etc.) must not have any damage.

⑥ All parts should be deburred before assembly; sharp corners and edges not shown in the drawings should be blunted (but this does not include the edges of sealing grooves).

⑦ Carefully and thoroughly clean all parts and components to meet cleanliness requirements, except for the sealing elements.

⑧ Cleaned parts and components should be dried before assembly; parts and components that cannot be assembled immediately should be wrapped or covered with plastic sheeting (film).

⑨ Inventory and register all assembly tools, process fixtures, and low-value consumables.

(2) Assembly

① General technical requirements for seal assembly. The function of seals is to prevent leakage or to ensure that the leakage amount meets the design requirements. Reasonable assembly processes and methods can guarantee the reliability and durability (service life) of the seals.

a. Inspect all parts according to the drawings, especially all chamfers, lead-in chamfers, and rounded edges, ensuring there are no burrs or flashings.  The surfaces of mating parts and seal grooves must not have tool marks (such as spiral marks, transverse marks, or chatter marks), scratches, dents, or corrosion.

b. Before assembly, all parts must be thoroughly and carefully cleaned and dried or wiped dry.  In particular, no cleaning fluid (oil) or other residues should remain in the seal grooves.

c. After cleaning, all parts should be assembled promptly; if not assembled immediately, they should be wrapped or covered with plastic sheeting (film).

d. Randomly check the specifications, dimensions, and surface quality of the seals according to the drawings, and take the required quantity at once; seals with surface contamination (such as oil stains, impurities, dust, or sand) should not be used directly for assembly.

e. Seals that have been assembled, disassembled, or used should generally not be reused for assembly, especially O-rings, coaxial seals, dust seals, and support rings (except for pre-assembly), retaining rings, etc.

f. A suitable amount of lubricating oil (grease) should be applied to all mating surfaces before assembly.

g. The lubricating oil (grease) applied during assembly must not contain solid particles or mechanical impurities (such as graphite or molybdenum disulfide grease). It is best to use the special lubricating oil (grease) specified by the seal manufacturer.

h. Rubber seals are best assembled at a temperature of (23±2) ~ (27±2)℃; seals stored at low temperatures must reach room temperature before assembly; seals (or parts containing grooves) that require heating for assembly should be heated with hydraulic oil at a temperature not exceeding 90℃, and should be assembled after they have returned to room temperature and cooled and contracted to their final shape.

i. Various seals must not be excessively stretched during assembly, nor should they be rolled into place, or subjected to localized strong pulling, pressing, twisting, folding, or forceful compression during assembly.

j. Repairing surface damage to parts is not allowed using sandpaper (or abrasive cloth). Fine oilstones may be used for grinding, and the repaired area must be cleaned thoroughly afterward.

k. No seals should be omitted or duplicated. The installation direction and position of the seals must be correct, and all assembled parts should be promptly integrated into the final assembly.

l. During final assembly, if pistons or cylinder heads (guide sleeves) need to pass through oil passages, keyways, threads, or relief grooves, protective measures must be taken to protect the seals (and other components) from damage.

m. After final assembly, dust plugs (caps) should be used to seal all oil ports of the components.  The seals and installation tools, including special tools and other low-value consumables such as rags, should be accounted for.

② Technical requirements for O-ring assembly.

a. The shaft and bore of the mating parts should have good coaxiality to ensure uniform clearance around the circumference.

b. During assembly, prevent the O-ring from being scratched, scuffed, or damaged. When inserting into the bore or onto the shaft end, there should be a sufficiently long guide cone (chamfer), and the intersection of the conical surface and the cylindrical surface should be rounded and have a smooth transition.

c. Apply an appropriate amount of grease to the groove before installing the O-ring. Before assembly, apply an appropriate amount of lubricating oil (grease) to all mating surfaces; after assembly, the mating parts should move freely, and the O-ring should be prevented from twisting or rolling.

d. For O-rings installed under tension, to allow the cross-section to return to a circular shape after stretching, after insertion into the groove, allow sufficient time before assembling the mating parts.

e. When installing or removing O-rings, use appropriate tools. The material and design of the tools should be carefully selected, and the ends and edges should be blunted.  Avoid using sharp, hard tools such as steel needles to pry the O-ring, preventing damage to its surface.

f. Used or removed O-rings and backup rings should not be reused for assembly.

g. Ensure the correct relative position of the O-ring and the backup ring.

③ Technical requirements for lip seal assembly.

a. Check the specifications, dimensions, and surface quality of the seal, especially ensuring that the lips (sealing edges) are free from damage or other defects; also check the dimensions and tolerances of all parts, surface roughness, chamfers, and fillets, ensuring there are no burrs or flash.

It is particularly important to distinguish between piston and piston rod seals, especially bore-type Yx seals and shaft-type Yx seals.

b. When assembling lip seals, ensure the correct orientation; for lip seals using backup rings, ensure the correct relative position of the backup ring and the seal.

c. Before installation, the mating surfaces of the components should be coated with an appropriate amount of lubricating oil (grease), and an appropriate amount of lubricating grease should be applied to the seal groove. The groove at the lip end of the lip seal should also be filled with lubricating grease, and air should be purged.

d. Installing lip seals generally requires special tools. The tools for disassembly and assembly can be manufactured according to the type recommended by the seal manufacturer. If the lip seal needs to be installed through threads, undercut grooves, or other seal grooves, special measures must be taken to protect the seal from damage.  The usual practice is to first install a special sleeve or add 3 (4) segmented clips in the seal groove.

e. Lip seals (or parts containing grooves) that require heating for assembly should be heated using hydraulic oil at a temperature not exceeding 90℃.  Assembly with the mating parts should only be performed after the seal has returned to room temperature and cooled and contracted. Water should not be used to heat lip seals, especially those made of polyurethane and polyamide materials.

f. The pressure ring, V-ring (with or without fabric), and support ring (elastic seal) of the V-ring seal must be correctly arranged and combined, and should not be adjusted too tightly during initial adjustment.

g. A pre-assembly should generally be performed after installing only the support ring to check the coaxiality of the mating parts and the assembly of the support ring. If possible, check the movement of the piston and piston rod to avoid rigid interference.

h. After assembly, the piston and piston rod should move smoothly throughout their full stroke without jamming or uneven resistance.

④ Technical requirements for coaxial seal assembly. Coaxial seals are composite seals consisting of a plastic ring and a rubber ring, with the plastic ring acting as the friction sealing surface. Therefore, they require a step-by-step assembly process.

The rubber ring must be assembled first; please refer to the technical requirements for O-ring assembly mentioned above for specific details.

a. The plastic ring of coaxial seals used for piston sealing generally requires heated assembly.  It is recommended to heat the plastic ring with hydraulic oil at a temperature not exceeding 90℃ until it becomes sufficiently elastic and pliable. It may be necessary to heat the piston as well, which facilitates the cooling and shaping of the plastic ring.

b. The assembly of the plastic ring of coaxial seals used for piston sealing generally requires special installation tools and shaping tools. These can be manufactured and used according to the type recommended by the seal manufacturer. If the assembly involves other seal grooves, relief grooves, etc., these should be considered in the design of the installation tool. The surface roughness of the shaping tool’s contact surface with the plastic ring should be comparable to the surface roughness of the mating part.

c. Coaxial seals assembled after heating must be cooled to room temperature together with the piston before assembly with the cylinder body; if the coaxial seal for the piston rod is installed using heating, it must also be cooled to room temperature before assembly with the piston rod.

d. The plastic ring of coaxial seals used for piston rod sealing can also be assembled after heating, but the plastic ring needs to be bent into a long concave shape before assembly. After assembly, a tapered mandrel shaping tool is generally required for shaping. Common problems to watch out for include: firstly, omitting the rubber ring; secondly, incorrect installation direction of the plastic ring, as stepped coaxial seals are unidirectional seals and have directional requirements during installation.

e. Before the piston is installed into the cylinder body (barrel tube) and the piston rod is installed into the guide sleeve or cylinder head, the angle and length of the chamfer at the end of the cylinder body and piston rod must be checked. The intersection of the conical surface and the cylindrical surface must be rounded and smoothly transitioned, and meet the surface roughness requirements specified in the drawing.

f. In a set of seals, the coaxial seal is generally installed first.

g. Pay attention to lubrication; dry assembly is strictly prohibited.

⑤ Technical requirements for support ring assembly. The support rings commonly used today are made of wear-resistant plastic material and are used to prevent the piston from colliding with the cylinder body, providing support and guidance.

The support ring can be considered a non-metallic bearing in a certain sense.

a. Check the groove dimensions and tolerances according to the drawings, especially the groove bottom and fillet radii. If possible, perform a pre-assembly to check the coaxiality and movement of each component. If the hydraulic cylinder end is equipped with a cushioning device, it is necessary to check whether the cushioning plunger interferes with or collides with the cushioning hole.

b. For support rings with a cut, the cut should be made and sized according to the requirements of global standard, but the width of the support ring cut should generally not be less than the recommended value.

c. For mass-produced products, a pre-forming tool for the support ring should be manufactured.

d. The support ring is generally the last component to be installed in a set of seals. If there are several support rings in a set of seals, their cut positions should be staggered during installation.

e. The support ring should be bonded and fixed by applying an appropriate amount of grease in the groove. Note that applying excessive grease is detrimental to the bonding and fixing of the support ring.

f. Before the piston is installed into the cylinder, the angle and length of the chamfer at the end of the cylinder must be checked. The intersection of the conical surface and the cylindrical surface must be rounded and have a smooth transition, and meet the surface roughness requirements specified in the drawing. Otherwise, the support ring is most likely to come out of the groove during piston installation.

g. The support ring should be installed with the same precision as bearing installation.  Hammering, squeezing, or grinding with sandpaper (cloth) to reduce the thickness of the support ring, or using thin sheets (films) between the groove bottom and the support ring to reduce the mating clearance, are not permitted.

h. Except for pre-assembly purposes, used support rings should not be reused for assembly.

⑥ Other Assembly Technical Requirements.

a. Ensure the correct positioning of all parts and components of the hydraulic cylinder according to the drawings and technical documents.

b. All connecting threads shall be tightened to the torque specified in the design; if not specified, refer to Table 1-75.

c. Important fixed joint surfaces shall be tightly fitted; any components installed or connected to the hydraulic cylinder shall be securely fastened.

d. After assembly, the hydraulic cylinder shall move freely, especially those designed with end cushioning plungers, without any movement interference or collision; all external connecting threads and oil port edges shall be free from damage.

e. Unless otherwise specified, the piston stroke length tolerance of general hydraulic cylinders shall conform to the provisions of global standard.

f. For hydraulic cylinders with stroke positioning or limiting devices, the stroke positioning or limiting deviation shall meet the technical (accuracy) requirements or relevant standards.

g. The surface of the hydraulic cylinder shall be clean, with smooth and natural rounded corners and flat welds, free from burrs or sharp edges.

h. The nameplate shall be clear and correct, and securely and neatly installed.

i. The paint on the surface of the hydraulic cylinder shall conform to the provisions of global standard, and the color of the topcoat can be determined according to user requirements. The piston rod, positioning valve stem surface, external machined surfaces of the oil inlet and outlet ports, and the nameplate shall not be painted. The plating shall be uniform and bright, without delamination, blistering, peeling, or rust.

j. The solid particle contamination level of the internal oil in general hydraulic cylinders shall not be higher than —/19/16 as specified in global standard; the solid particle contamination level of the internal oil in servo hydraulic cylinders shall not be higher than 13/12/10 as specified in global standard.

k. Anti-rust measures shall be applied to the supporting parts and other exposed machined surfaces of the hydraulic cylinder.

l. Exposed oil ports of the hydraulic cylinder shall be covered with oil-resistant dust caps, and protective covers shall be added to the exposed threads of the piston rod and other connecting parts.

m. Ensure that the sealing performance (including minimum pressure starting performance) meets the technical requirements.

n. Carefully check and count all assembly tools, process fixtures, and low-value consumables.  No gaskets or other items not specified in the drawings and technical documents are allowed to be installed in or incorporated into the sub-assemblies or final assembly of the hydraulic cylinder. Ensure that no parts are missing.

Note:

1. In addition to the statement “The hydraulic cylinder should move freely after assembly,” further requirements can be found in next Section 1.3.13 of this Hydraulic cylinder engineering newsletter.

2. Regarding the technical requirements for nameplates (identification plates), the following more specific provisions are found in other standards: “A clear and permanent mark or nameplate should be placed in a suitable and conspicuous location on the hydraulic cylinder, or a pre-fabricated nameplate should be fixed in the position specified in the drawing. The nameplate should be clear, accurate, and flat.”

3. Threaded connections should be provided with appropriate anti-loosening measures (to prevent relative rotation of the threaded pair). If set screws are used for anti-loosening, the set screws themselves should also have anti-loosening measures, such as applying adhesive (but care should be taken to select an adhesive suitable for the operating temperature range of the hydraulic cylinder); for screw holes that need to be (machined) on the screw shaft, refer to standard “Holes for Fixing Screws on Shafts”.